ENH: Builder: strip trailing whitespace from field values; closes #297

- warn and remove trailing whitespace in str or code components
  because it can cause problems, seems unlikely to be intended (except for Text)
- don't touch Text fields, e.g., user might actually want only whitespace
- only happens once, at compile time
  changes the actual experiment: removes not just ignores whitespace
- for now, warnings are just printed; logging.warning() not working